Generally, MIMO is the method of using a plurality of transmitting antennas and a plurality of receiving antennas, by which data transmission/reception efficiency can be enhanced. In particular, a transmitting or receiving side of a wireless communication system uses a plurality of antennas to increase capacity or enhance performance. In this disclosure, the MIMO can be called ‘multiple-antenna’.
In order to receive one whole message, the MIMO technique does not depend on a signal antenna path. Instead, the MIMO technique completes data by gathering data fragments received via plural antennas. If the MIMO technique is used, a data transmit speed (data rate) is raised or system coverage can be increased by securing a specific data rate. Moreover, this technique is widely applicable to a mobile communication terminal, a relay and the like. According to the MIMO technique, it is able to overcome the throughput limit of the conventional mobile communication that adopts a single antenna.
FIG. 1 is a block diagram of a general MIMO communication system. Referring to FIG. 1, NT transmitting antennas are provided to a transmitting side, while NR receiving antennas are provided to a receiving side. If each of the transmitting and receiving sides uses a plurality of antennas, the theoretical channel transmission capacity is increased bigger than that of the case that either the transmitting side or the receiving side uses a plurality of antennas. In this case, the channel transmission capacity is increased in proportion to the number of antennas. Therefore, a data rate is raised and frequency efficiency is enhanced. The data rate according to the increase of the channel transmission capacity can be theoretically raised by an amount resulting from multiplying a maximum data rate R0 of the case of using a single antenna by an increase rate Ri. In this case, Ri is a smaller value between NT and NR.
For instance, in MIMO communication system using 4 transmitting antennas and 4 receiving antennas, it is able to obtain a data rate 4 times greater than that of a single antenna system theoretically. After the theoretical capacity increase of the multiple antenna system has been proved in the mid-90s, many efforts are ongoing to research and develop various techniques to realize the substantial enhancement of data rate. And, standards of the 3rd generation mobile communication and various wireless communications have already reflected some of these techniques.
The MIMO techniques can be classified into a spatial diversity scheme of raising transmission reliability using the same symbols through various channel paths and a spatial multiplexing scheme of enhancing a transmission rate by simultaneously transmitting a plurality of different data symbols using a plurality of transmitting antennas. Moreover, by combining the two schemes appropriately, it is able to obtain advantages of the two schemes properly.